This is the Alexandria Aqueduct, or what is left of it, in Georgetown, Washington, D.C. According to Mike High's The C&O Canal Companion (1997), the aqueduct once connected the C&O Canal to another canal (not named in the book) on the Virginia side across the Potomac River, which is visible on the right (the Key Bridge is in the background). The C&O Canal is towards the left of the aqueduct in this picture.
I was there last Friday. I noticed that parts of the walls of the aqueduct were wet from water seeping out from between the rock blocks (it had rained the day before). A close inspection revealed tiny snails crawling on a section of the wet wall partially covered with red ivies (towards the lower left in the photo). A closer inspection revealed that the tiny snails were Vallonia.
They are either Vallonia pulchella or V. excentrica, the 2 being somewhat hard to distinguish. Their shell diameter is ~2.3 mm.
The next time I am there I will check again to see if the snails are always out on the wall.